RNA polymerase (Pol) I (one of three eukaryotic nuclear RNA polymerases) is devoted to the transcription of the ribosomal DNA genes, which are found in multiple arrayed copies in every eukaryotic cell. These genes encode for the large ribosomal RNA precursor, which is then processed into the three largest subunits of the ribosomal RNA, the 18S, 28S, and 5.8S RNAs. In human cells the rDNA gene clusters are localized on the short arm of the five pairs of the acrocentric chromosomes. The rRNA promoter has two essential and specially spaced sequences: a CORE element and an upstream control element (UCE, also called UPE). The CORE element of the human promoter overlaps with the transcription start site, extending from 20 to 45, and is required for specific initiation of transcription.
The polymerase is a multisubunit complex, composed of two large subunits (the most conserved portions include the catalytic site that shares similarity with other eukaryotic and bacterial multisubunit RNA polymerases) and a number of smaller subunits. Under a number of experimental conditions the core is competent to mediate ribonucleic acid synthesis, in vivo however, it requires additional factors to select the appropriate template. In humans the RNA transcript (45S) is approximately 13,000 nucleotides long. Before leaving the nucleus as assembled ribosomal particles, the 45S rRNA is cleaved to give one copy each of the 28S rRNA, the 18S rRNA, and the 5.8S rRNA. Equal quantities of the three rRNAs are produced by initially transcribing them as one transcript.
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UBF-1 binds directly to the CORE and UCE elements of the ribosomal DNA promoter. This binding is mediated by the HMG boxes (primarily HMG box1). Phosphorylation may play a role in the modulation of UBF's DNA binding activity, as well as in subsequent steps. UBF is thought to bind DNA in a conformation specific manner (as opposed to a sequence specific manner). The binding of UBF to the minor groove of DNA induces strong DNA bending.
Phosphorylation of UBF-1, bound to the promoter, activates UBF-1 and recruits SL1, and eventually polymerase. This phosphorylation of UBF-1 by Erk1, has been shown to both weaken the binding of UBF-1 to DNA and to activate transcription (the authors of the paper showing these data suggest that loosening the binding of UBF-1 with the promoter may somehow promote transcription initiation). Though not definitively worked out phosphorylation of UBF-1 by Erk1 plays a role in the activation of the UBF-1:rDNA complex.
Human SL1 is a four subunit complex composed of the TATA-binding protein (TBP) and three TBP-associated factors (TAFs): TAF(1)110, TAF(1)63, and TAF(1)48. Note that none of these three TAFs for Pol I show any homology to the Pol II or Pol III TAFs. TAFs SL1 is a species specific factor.
Acetylation of the TAFI63 subunit of SL1 by PCAF stimulates the association of TAFI63 with DNA and stimulates pol I transcription in vitro. Conversely, deacetylation by the NAD+-dependent deacetylase Sir2 represses pol I transcription.
Human SL1 does not bind to DNA itself, rather it is recruited to the rDNA promoter through a physical interaction with UBF-1. Phosphorylation of UBF-1 within the carboxy-terminal region is required for SL1 binding. SL1 consists of TATA-binding protein (TBP) and three associated factors (TAFIs). SL1 has no sequence-specific DNA binding activity its recruitment to the promoter being mediated by specific interactions with UBF. Once bound the SL1 complex makes direct contact with the DNA promoter and guides promoter-specific initiation.
Studies to identify the mechanistic relationship between SL1 and UBF-1 have indicated that the interaction between UBF-1 and SL1 is regulated by tumor suppressor proteins such as Rb and P53, although it has also been proposed that Rb prevents UBF-1 from binding to DNA itself.
Composed of Acetylated SL1, phosphorylated UBF-1 bound the rDNA promoter as well as the active RNA polymerase holoenzyme, rrn3 and TFIIH the transcription initiation complex is complete. The assembly picture is incomplete, as the point at which TFIIH joins the complex is unknown, though by the time that this complex is formed TFIIH is present (it has been included at this step for completeness). This forms the transcriptionally active enzyme, that is capable of initiating transcription from the rDNA promoter.
Upon transcription initiation it is thought that RRN3 is inactivated and dissociates from the Loss of Rrn3 from the RNA Polymerase I promoter escape complex. SL1 and UBF are thought to remain bound to the promoter for multiple rounds of transcription initiation
At the beginning of this reaction, 1 molecule of each of POLR1A (RPA190, A194), POLR1B (RPA135), POLR1C (RPA40), POLR1D (RPA19), POLR1E (PAF53, RPA49), POLR2E (RPB5), POLR2F (RPB6), POLR2H (RPB8), POLR2K (RPABC4, RPB12), POLR2L (RPB10), TWISTNB (RPA43), CD3EAP (CAST, PAF49), and ZNRD1 (RPA12) are present. At the end of this reaction, 1 molecule of 'RNA Polymerase I Holoenzyme (Human)' is present. This reaction takes place in the 'nucleolus'.
At the beginning of this reaction, 1 molecule of 'elongating pre-rRNA transcript', and 1 molecule of 'NTP' are present. At the end of this reaction, 1 molecule of 'elongating pre-rRNA transcript' is present.
This reaction takes place in the 'nucleolus' and is mediated by the 'DNA-directed RNA polymerase activity' of 'RNA Polymerase I promoter escape complex'.
As inferred from mouse cell models, the Transcription termination factor (TTF1, also known as TTF-1 and TTF-I) binds an 18 base pair sequence element known as the Sal Box found in multiple copies in the nontranscribed spacer downstream of the 28S rRNA coding region. This element is the termination signal for ribosomal gene transcription. Binding of TTF1 mediates the pausing of the elongating transcription complex. TTF1 has a relatively low affinity for purified DNA but binds cooperatively to chromatin. Oligomers of TTF1 interact in trans to bind adjacent intergenic regions and form loops of the rDNA. Binding of TTF1 to the Sal Box is also influenced by interaction of TTF1 with TIP5 and possibly other proteins.
PTRF binds the quaternary complex and mediates the dissociation of paused complex. PTRF interacts with the RNA polymerase I largest subunit (p194), TTF-I and the U-rich 3' end of the nascent pre-rRNA.
Dissociation of paused ternary complexes requires the Polymerase I-transcript release factor (PTRF) a leucine zipper protein. PTRF is capable of dissociating ternary Pol I transcription complexes, interacting with both TTF-I and Pol I to mediate the release of both Pol I and nascent transcripts from the template.
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DataNodes
SL1:PhosUBF-1:rDNA
promoterI/Nascent Pre rRNA Complex:TTF-I:Sal
BoxPolymerase
I:rRNATranscript:TTF-1:Sal Box ComplexTranscription
Initiation complexpromoter escape
complexAnnotated Interactions
SL1:PhosUBF-1:rDNA
promoterSL1:PhosUBF-1:rDNA
promoterI/Nascent Pre rRNA Complex:TTF-I:Sal
BoxI/Nascent Pre rRNA Complex:TTF-I:Sal
BoxStudies to identify the mechanistic relationship between SL1 and UBF-1 have indicated that the interaction between UBF-1 and SL1 is regulated by tumor suppressor proteins such as Rb and P53, although it has also been proposed that Rb prevents UBF-1 from binding to DNA itself.
This reaction takes place in the 'nucleolus'.
This reaction takes place in the 'nucleolus' and is mediated by the 'DNA-directed RNA polymerase activity' of 'RNA Polymerase I promoter escape complex'.
Polymerase
I:rRNATranscript:TTF-1:Sal Box ComplexPolymerase
I:rRNATranscript:TTF-1:Sal Box ComplexTranscription
Initiation complexTranscription
Initiation complexpromoter escape
complexpromoter escape
complexpromoter escape
complexpromoter escape
complex